I would brace the bb against something solid like a block of wood, get someone to hold the frame for you so it doesn't move then use a suitable cold chisel and hammer against one of the grooves you have already made remembering you have to turn it anticlockwise to loosen it, maybe give it a good soaking with plusgas or a good quality releasing spray (wd40 is no good)
Some good info here,
Removing the cup using nut and bolt and a few washers
The bolt is put through the cup from the inside of the bottom bracket, and the nut screws onto it from the outside. There should be washers on both sides of the cup. You will need a socket wrench and a short extension to tighten the bolt from inside the bottom bracket, and a box wrench for the nut.
A large socket wrench will not fit into the cup, so you will need some small-diameter washers to space the head of the bolt out so that the socket wrench can reach it. This is what I use the four split lockwashers for. I didn't need lockwashers as such, but the readily available flat washers were too large in outside diameter to fit into the cup.
To remove a right-threaded fixed cup (French or Italian), tighten the bolt with the socket wrench from the inside of the bottom bracket, holding the nut with the box wrench. Once the bolt is as tight as can be, keep on tightening it, until the cup screws itself out. Once the cup starts to move, turn both wrenches together.
To remove a left=threaded (British, Swiss or Raleigh) fixed cup, hold the bolt with the socket wrench and turn the nut with the box wrench.
Use the same tool the opposite way to install the new fixed cup.
You might worry about damaging the cup, but this is not likely to happen. The washer doesn't usually come into contact with the bearing surface. Even if it did, bearing cups are made of extraordinarily hard, heat-treated steel, much stronger than that used to make washers.
http://www.sheldonbrown.com/tooltips/bbcups.html
